Segfault on x86_64

[gcsbt]

Hello,

I encounter a crash on a x86_64 machine. It looks like the rdx register is dereferenced while being invalid:

Program received signal SIGSEGV, Segmentation fault
0x000055555558aa2c in _ ()
(gdb) x/6i $pc-20
   0x55555558aa18 <_+56>:    rdtsc
   0x55555558aa1a <_+58>:    shl    rdx,0x20
   0x55555558aa1e <_+62>:    or     rax,rdx
   0x55555558aa21 <_+65>:    mov    QWORD PTR [rcx],0x1
   0x55555558aa28 <_+72>:    mov    QWORD PTR [rcx+0x8],rax
=> 0x55555558aa2c <_+76>:    movzx  eax,BYTE PTR [rdx+0x14]
(gdb) i r $rdx
rdx            0x227bd00000000     606642655723520

I guess the reason is rdx is not marked as an out register in start() and stop() functions, and the compiler assumes it isn't modified.

Suggested fix (untested): add out("rdx") _ to the asm! macros if you don't want to rely on core::arch::x86_64::_rdtsc.